This invention relates to a loop network system of the token controlled type, and more particularly to a system of the type above described which is suitable for controlling acquisition of the right to transmit data.
In a loop network system of the token controlled type, a frame for controlling the right to transmit data, which frame is called a token frame, is transmitted to a loop transmission line for controlling the shift of the data transmission right among a plurality of nodes connected to the loop transmission line. The conventional protocol used for allocation of the data transmission right to one of such nodes by transmission of the token frame is broadly classified into two modes of operation. According to the first mode of operation, the node nearest to a node which has transmitted the token frame has the highest priority right, and the node remotest from the node which has transmitted the token frame has the lowest priority right. (That is, in this mode of operation, the priority order is not based on the logical level.) According to the second mode of operation, a priority level bit is provided in the token frame, and a node cannot acquire the data transmit right unless the level of a frame to be transmitted from the specific node is higher than the priority level provided in the token frame when the specific node has received the token frame.
Various problems as described below arise in the case of the prior art system when, for example, a channel unit of a central processor is connected together with high-speed input/output units to an optical loop network, so that the input/output operation of the input/output units can be controlled under command of the channel unit. Herein, the node corresponding to the channel unit is called a master node, and the node corresponding to the input/output unit is called a slave node.
(1) Generally, the frame transfer request transmitted from each of the slave nodes to the master node includes various levels such as a data transfer request from the input/output unit, a report of the end of operation of the input/output unit, a report of an interrupt occurring independently of the command from the master node, a report of trouble having occured in the input/output unit, etc. The master node must decide which level of the transfer request from the slave node is to be accepted depending on its own internal status and the importance of the request. However, the master node to which such a request is transmitted cannot freely select the request level.
(2) The master node is determined already and, to that node, the frame transfer request is to be transmitted from the slave node. It is therefore necessary that the frame from the slave node is to be transmitted to the master node, only when the master node is free. That is, it is necessary for the slave node to carry out acquisition of the data transmission right only as to the frame for which the data transmission right is allotted from a predetermined master node. However, according to the prior art system, there are no means permitting selective acquisition of the data transmission right, and the prior art system is not suitable for the control of the data transmission right in the case where a plurality of channel units are connected to a single loop.
(3) When data transfer at a high speed is requested between one of the slave nodes forming a pair with the master node, data cannot be transferred at high speed according to the prior art system, since only a half-duplex data communication linkage can be established due to the fact that only one of the nodes has the data transmission right.